ENGL Thunder 50W repair

[vega77]

Im currently repairing an engl thunder 50W from about 2000.

It had a broken plate resistor for the phase inverter.

There has been several repairs and mods over the years.
I did find a schematic, but even some of the orginal stuff doesnt correspond 100%

Theres a bridge recifier for heater supply to the preamp with 2x4700uf 16V on the output.
Power tubes has ac for heaters.
This looks orginal but is not on my schematic.

HT+/B+ is rectified via a bridge rectifier made with 4 singel diodes.
In paralell with each diode theres a cap.
Im not sure why the caps are there or if its orginal.

My PCB is labeled 325TOP-A0
I would be greatful if someone had the correct schematic.

[PRR]

Welcome.

All of this sounds like details discovered in testing and not copied-back to the original drawing.

Cap across rectifier may protect against spikes and radio interference. Filtered DC for preamp heaters is a standard patch for bad hum from AC heat lines on a printed circuit board.

[glass54]

Hi vega,
I have some Engl sch but not the Thunder.
Looking at Engl Fireball (50W 2x6L6 tubes) schematics, the main HT supply 4x1N4007 diodes have 1n caps fitted.
For Preamp tubes, the DC power supply has 2x6800uF caps in parallel, after Bridge rectifier (4x1N4007) wired to 6.3VAC.
As PRR suggested, this was probably a production upgrade.
Hope this helps and good luck finding the exact schematic.
Regards
Mirek

[vega77]

Thanks for input.

Got it wired up for testing now.
It is running away on bias current, bias voltage at controlgrid unloaded was -55V.
Is that a bit high ?

[sluckey]

It is running away on bias current, bias voltage at control grid unloaded was -55V.

I would expect the bias current to be very low with -55v on the grid. Are you sure you did that correctly?

[glass54]

Can you post the schematic that you have, please. It will be easier to assist you.
I assume you have Cathode resistors? on the output tubes, possibly 4R7?
It will be easy to monitor output tube current (Divide mV across resistor by 4R7?) Strange value 
Regards
Mirek

[WimWalther]

Strange value 

Maybe they were going for 42mA = 200mV? =)

But you're right, current-viewing Rk are typically 1R (or lower) 1% parts. They might also be using them as shock absorbers, so a hard tube short doesn't take out the OT so easily? 5R is about right for that.

[glass54]

I like to keep it simple, 1R with my Fluke 79 multimeter on mV. (ie KISS principle and What you see is what you get).
Now we can raise another can of worms, fitting Cathode resistors (Push_Pull design) in the range of 4R7 to 47R introduces a little local feedback, maybe a little magic in the HiFi world, but do we need it for guitar? 
Kind regards
Mirek

[WimWalther]

I like to keep it simple, 1R with my Fluke 79 multimeter on mV. (ie KISS principle and What you see is what you get).

Some of the younger folks around here might not know this, but Dynaco was well-known for picking specific, seemingly odd values for their output tube cathode resistors - 15R6 in the Stereo 70, 11R2 in the Mk.III, etc.

But they had a very neat idea. See, accurate mV meters weren't cheap & common then, as they are today. But zinc-carbon dry cells were everywhere, and new ones were very reliably at about 1.56V. Soo..

Dynaco chose resistors that would drop exactly 1.56V when bias was set right on-spec, and one could use a common dry cell to calibrate their meter for 1.56V. For the ST70 this was 100mA (15.6 x 0.100 = 1.56V) and 139mA for the Mk.III (11.2 x 0.139 = 1.56V).

So even if the volrmeter were way off, you still knew where it read 1.56V and thus your bias was correct. You could also connect the battery in series with the meter (on low DC current range) and set bias by nulling the meter.

Pretty clever, really.